It is characteristic of some alcoholic beverages especially stout and traditional ales and beers to generate a foamy head of gaseous bubbles during the dispensing of the beverage into a glass and to consume the drink with this head evident upon the liquid. The source of gas for the bubbles is the gases dissolved in the beverage which are caused to break out of solution through a nucleation process. When dispensing from the bar this nucleation process has been stimulated by forcing the beverage under very high pressure through small nozzles which create sufficient sheer force to stimulate gas nucleation.
It is also known that if nitrogen is dissolved in such beverages, the bubbles are smaller, more stable and are perceived as creamier than when only carbon dioxide is present. It has therefore become common practice to add nitrogen to certain beers, ales and stouts. To maintain the nitrogen in solution, nitrogen has been used in the gas over pressure dispensing systems for dispensing such beverages so as to promote a stable and creamy head.
It has also become commonplace to add nitrogen to canned alcoholic beverages as aforesaid and to pressurise the can with nitrogen to the extent of adding liquid nitrogen during filling, so that after the can is sealed the evaporating dose of liquid nitrogen will increase the internal pressure typically to two atmospheres or more.
The can pressurisation has enabled thinner walled cans to be used and the use of non-oxidising gas for the pressurisation (after purging the can and contents of all oxygen), has ensured that oxygen will be absent from the interior of the can. If nitrogen is used, it will be taken up by, and become dissolved in, the beverage, so that if the latter can be stimulated to give up the nitrogen on dispensing, a rich creamy head of nitrogen bubbles will be formed on the beverage.
Various techniques have been adopted to stimulate the bubble formation on dispensing from such a pressurised can.
Early attempts are described in GB 1266351 particularly in relation to FIG. 3, wherein a secondary chamber is defined within the can which is adapted to retain a charge of gas under pressure, which discharges into the beverage through a fine orifice, driven by the pressure difference arising immediately after the can is opened to atmospheric pressure by the consumer.
Practical difficulties with this described technique apparently prevented commercial application for many years. The problems included the complexity and cost of modification to standardise packaging, the necessity to develop specialised can or bottle filling equipment for non-standard packages, the necessity to minimise oxygen in the package usually causes a beverage to change in flavour, the requirement that there should be minimal reduction in effectiveness of the gassing device caused by temperature and pressure fluctuation which can arise during transportation and distribution, and that the end product appearance and taste should be independent of the procedure used by the consumer to open and pour the packaged beverage.
Some of these difficulties were overcome by the use of a secondary chamber in the form of a capsule disclosed in EP 227213A2 in which the secondary chamber is pressurised from the primary container and its contents discharged through a permanently open orifice in the side wall of the capsule into the beverage when the can is opened.
Problems associated with the fitting and retention of such capsules resulted in other proposals such as described in GB 2211813A (Price) in which the secondary chamber is formed by an apertured diaphragm which divides the interior of the can into an upper larger part and a smaller lower part. It had already been proposed in EP 227213 to used an oversize can so as to provide a headspace in the can above the beverage. This not only provided space into which the creamy head could rise but also allowed for the additional volume of the capsule (or separate compartment such as proposed in GB 2211813A Price) and for the extra beverage required to compensate for any beverage trapped in the capsule of EP 227213 or the lower compartment of GB 2211813A Price.
The quantity of beverage in the secondary compartment is clearly minimised by inverting the can as has been commonplace between filling and pasturisation since the introduction of the two-piece can following the published recommendation of the UK can manufacturer concerned as early as 1981. This inversion causes the orifice in the secondary compartment to communicate with the gaseous headspace, as described in GB 2211813A Price.
Whilst the Price design allows all the beverage to drain from the secondary chamber, this is only achieved if the can is not only inverted during processing but is then left inverted until just before being opened. Price suggested that to this end the can should be printed "upside down" so that there would be a chance that the purchaser would place the cans in their inverted state whilst awaiting use. However there was no guarantee that the cans would be so stored, in which event the lower compartment would be filled with beverage.
Although this would be under pressure and would jet through the aperture or apertures in the diaphragm of Price when the can was opened to atmospheric pressure, the results of such jetting of beverage did not result in any useful head formation and unlike the capsule of EP 227213A2 the diaphragm of Price could not allow a pocket of gas to be trapped to be discharged instead of (or as well as) some of the beverage.
It can only be concluded that the Price proposal was not taken up since it could not be guaranteed that the consumer would store the can upside down and invert sufficiently quickly before opening, to prevent any of the beverage from transferring below the diaphragm. Additionally there was no significant advantage to the manufacturer since the canning of the product still had to provide excess beverage over and above what the can was stated to contain in case the can was not stored the correct way up and thereby trapped beverage below the diaphragm.
EP 360375A1 describes a further development which combines the advantage of the capsule of EP 227213A (in that gas can be trapped by the device when the can is upright) with the price proposal for a diaphragm (so as to avoid the capsule fitting and retention problems). Clearly there will always be a charge of gas trapped below the domed diaphragm of EP 360375A1 which can be maximised (and the volume of beverage minimised) if the can is inverted and left so inverted as taught by Price.
EP 360375A1 describes an alternative method of constructing a domed diaphragm and an alternative filling process in which the can is filled upside down, to ensure the compartment will be filled with gas before the can is turned over to stand on its base with the domed compartment at the bottom. Since the specification envisages dosing with liquid nitrogen the pressure of the gas in the section of the can between the lid and the domed diaphragm will be greater than atmospheric very shortly after the can is sealed and this will ensure that a good charge of high pressure gas is available below the domed diaphragm when the can is subsequently inverted.
However as shown in the drawings of EP 360373, there is still a tendency for beverage to displace some of the gas at least up to the level of the aperture. As a consequence although the high pressure gas trapped below the dome will be jetted into the beverage (together possibly with some of the beverage) so as to form the desired head when dispensed, the beverage below the aperture will remain in the base of the can in the same way as it remains below the level of the aperture in the capsule of EP 227213.
The trapped beverage represents lost revenue which can be significant in the case of alcoholic beverages, particularly if tax is levied on the volume of beverage poured into the can rather than on the volume which can be poured out.
The loss of revenue can be mitigated in two ways:
1. reduce the cost of the gas-storing head-producing device and the cost of inserting it into the can, and/or PA1 2. reduce the volume of beverage which can be trapped within the gas producing device.
PCT/GB90/01806 (Whitbread) addresses the second option by proposing a sealed gas containing device into which beverage cannot ingress and which only opens to communicate with the beverage after the can has been opened and depressurised, so that there should be no reverse transfer of beverage into the capsule as gas leaves it. However the cost of production of such devices is not inconsiderable and the complexity of the pressure sensitive mechanism of the device to release the gas only when the can is opened, means that in practice there has been a relatively high failure rate, resulting in poor or even no head formation on beer dispensed from faulty cans.
EP 520646A1 describes a modified construction of the type of capsule described in EP 227213 which is also charged with gas from the headspace following headspace transfer by means of can inversion, as described in UK 2211813 Price.
The design of the capsule allows any beverage which has entered the capsule to be collected below the level of the aperture, so there is little tendency for it to be ejected ahead of or instead of the gas, provided the can is opened whilst upright. In this respect the device has the same advantage as the Price design, in that as with the Price device, no energy is wasted in ejecting beverage into the contents of the can, and it is gas only which is ejected.
It is suggested that the ingress of beverage into the capsule of EP 520646A1 can be reduced by inverting the can as quickly as possible after filling, but this seems to be nothing more than a restatement of the Price technique, in which, if the can is inverted immediately after filling and sealing, no beverage will have entered the lower chamber of Price, and in any event it has been commonplace to invert filled cans on canning lines within a few seconds of the final seaming of the can, for the reasons already mentioned.
The design of the capsule in EP 520646A1 is in many ways also similar to that shown in GB 1266351 in that the orifice by which the secondary chamber communicates with the rest of the can points downwardly towards the base of the can, so that an air/liquid lock is formed and there will be little tendency for beverage to displace any of the trapped gas, unless the can is tilted. The side tube design of GB 1266351 may of course include a small volume of beverage if there is a liquid exchange as during pasturisation, or thermal cycling of the can during storage, and in this respect the capsule of EP 529646A1 is better than that of GB 1266351 in that there is no slug of beverage to force out ahead of the gas charge. However the EP 520646A1 capsule suffers from a further problem in that, if as is likely to occur, some beverage does enter the capsule, since if the can is tilted with the orifice is on the underside of the capsule, any beverage trapped in the capsule will tend to occupy the position such as shown in FIG. 2 of EP 520646A1, except that in this case the beverage will now overlie the orifice 12, which in FIG. 2 is conveniently shown remote from the pool of liquid. Clearly if the liquid within the capsule does cover the orifice 10, the claimed advantage of an initial jetting of gas will be lost and because of the variableness of the volume of liquid in the capsule and the possibility that quite a large volume of liquid must be expelled from the capsule before the gas can escape, energy in the gas stored in the capsule will be lost as the liquid is expelled.
The capsule design of EP 520646A1 does not therefore solve the problems identified above regarding variability in the volume of retained beverage in the capsule and variability introduced into the gas jetting characteristic if a significant quantity of beverage occupies the interior of the capsule and can cover the exit orifice during pouring.